For the navigation of an aircraft along a route to a specified destination, the pilot makes use of various types of on-board equipment, including sophisticated navigational computer systems which are capable of providing guidance information such as heading, altitude, geographical position, ground speed, and the like. To obtain a complete picture of the flight path of the aircraft the pilot also makes use of an aeronautical chart that contains geographical data including elevation and cultural terrain features such as rivers, lakes, natural rock formations and vegetation, as well as man-made cultural features, such as roads, towns, and significant structures, each uniquely identified in the map by a specific color for the purpose. For example, a lake may be shown on the map using a deep or dark blue color, while a river or stream feeding the lake may be a light blue color, and a surrounding forest a dark shade of green.
In order to incorporate the aeronautical chart with other aspects of the aircraft's guidance equipment, the chart is normally presented to the pilot via a cockpit display, such as a cathode ray tube (CRT) display, with various navigation indicators superimposed on the displayed chart. Because the pilot is normally interested in observing only a prescribed portion of the map which represents an area of terrain immediately surrounding the geographical position over which the aircraft is currently flying, yet the map display must be capable of providing coverage for hundreds of miles of an overall flight path, the provision of an electronic data base from which such map data may be accessed could involve an on-board memory storage capacity that would be prohibitive due to space and weight restrictions placed on airborne system components. To satisfy these requirements, aircraft display digital data bases are typically stored in a compressed format with a resolution of some number of plural bits per pixel of the CRT display, the storage mechanism consisting, for example, of a magnetic storage medium, such as a magnetic tape or disc. In the reconstruction processor on-board the aircraft, the stored compressed data is decompressed and the original image reconstructed for display to the pilot. Unfortunately, compression/reconstruction techniques that have been proposed to date have limited resolution capability and/or are extremely complex to implement, so that they suffer from a substantial airborne storage and processing penalty that limits their practical use.